The friction between valve guide and valve stem is essentially Coulomb friction, so that friction is proportional to the side loads between these parts. In addition, minimization of valve stem side loads on the valve guide will act strongly to reduce wear of the valve guide in service. It is, therefore, desirable to reduce the side loads due to asymmetric loading of the spring with respect to the valve spring retainer.
The coil springs which operate in engines do not put their loads on the valve spring retainer uniformly, and it is not practical to manufacture them so that they do. However, if the valve spring retainer is free to rotate to a small degree out of perpendicular to the valve guide, it can adjust so as to essentially eliminate torques and side loads on the valve guide due to this imperfection in the coil spring. This can be done with a spherical valve lock-valve spring retainer arrangement as follows.